The present invention relates generally to prostate brachytherapy (radioactive seed implantation), and more particularly concerns a method and system for determining prostate boundaries in transrectal ultrasound images.
Prostate adenocarcinoma is the most frequently diagnosed cancer in men and remains the second leading cause of death of American men. The reported incidence of prostate cancer has been increasing in the past ten years due to increased life expectancy, prostate specific antigen (PSA) screening, and improved diagnostic techniques, including transrectal ultrasound (TRUS), CT and MRI. Autopsies of individuals dying of other causes show prostate cancer cells in 30% of men at age 50; by the age of 80 years, the prevalence is 60%. Further, prostate cancer can take up to 10 years to kill the patient after initial diagnosis. Prostate cancer is newly diagnosed in over 100,000 men in the US each year, and over 40,000 will die of the disease. These trends are expected to continue in the near future.
There are four standard treatment regimens for prostate cancer: (1) watchful waiting, (2) radical prostatectomy, (3) external beam radiation; and (4) interstitial brachytherapy. The rising incidence of earlier stage prostate disease in younger patients with longer life expectancies has brought both treatment effectiveness and the quality of life into focus. As an effective treatment option, the rapid development of transperineal prostate brachytherapy has provided an alternative for patients seeking to preserve their prostate function as well as control the cancer. Interstitial brachytherapy, which can be performed as a single outpatient treatment, is recommended for patients with early stage cancer. With a high likelihood of disease located outside the prostate, the doctors often follow external beam radiation treatments with interstitial brachytherapy. Because prostate brachytherapy is associated with a lower incidence of incontinence, impotence and rectal injury, it is emerging as a medically successful, cost-effective outpatient treatment in treating localized prostate cancer.
The interstitial brachytherapy technique is well known and is described in detail in many publications. Interstitial brachytherapy involves the accurate placement of radioactive seeds into the prostate gland according to a predetermined dosimetry plan, which is optimized to cover the whole prostate with enough dosimetry. The implant needles, guided by a grid template, are inserted into the prostate gland under the guidance of a transrectal ultrasound probe, which is inserted into the rectum of the patient. Then a seed applicator or spacer is used to locate the seeds at special positions.
For calculating the optimized dosimetry plan, a prostate volume study using the transrectal ultrasound probe is an important step for successful execution of the brachytherapy procedure. In the volume study, the patient lies on his back, the ultrasound probe is inserted into the rectum with the aid of a stabilizing or stepper apparatus and transverse cross-sectional images of the prostate are acquired at fixed intervals, e.g., 5 mm increments from the base (superior) of the gland to the apex (inferior). Currently, the prostate boundary is manually outlined by a clinician in each transverse cross-sectional ultrasound image. The overall volume of the prostate is determined using well-known step section planimetric techniques. The boundaries of the prostate obtained during the volume study not only result in an accurate determination of the size and shape of the prostate, but also provide important information for developing the radiation dosimetry plan. The end result of the computerized dosimetry plan is an accurate map for placing the seeds within the gland.
The ultrasound probe, which is inserted in the patient""s rectum, is used to obtain the transverse cross-sectional images of the prostate during the initial prostate volume study. Though the transrectal ultrasound provides a better quality image of the prostatic tissue than CT and MRI, the visibility of the prostate boundaries is typically poor for the following reasons:
(1) The pelvic musculature and rectal wall generate strong echoes, which result in linear artifacts in the images, which give the false appearance of a boundary.
(2) The incident ultrasound beam is scattered in random directions, producing artifacts, such as missing edges.
(3) The top part of boundary is positioned at a considerable distance from the ultrasound probe and, hence, greater attenuation of the ultrasound energy results in poor contrast.
(4) The boundary located in the far field of the ultrasound image is greatly degraded due to much lower contrast.
(5) Part of the boundary is outside of the imaging window so that they are invisible in the transverse section images.
To overcome such deficiencies, typically, the practitioner must manually draw the prostate and rectum boundaries. The manual drawing of the prostate and rectum boundaries takes a longer time than is desirable (nearly 10-20 minutes for a whole series) and brings inconvenience to the operation. Also, each clinician will give different definition of the prostate boundary because of the various experiences and knowledge; that is, drawing prostate boundaries is user-dependent.
It will be readily apparent from the above that a need exists in the art to reduce the time requirement and subjectivity in the determination of prostate and rectum boundaries. It is therefore a primary object of the present invention to provide accurate, stable, and automatic segmentation of prostate boundaries prior to beginning the seed implant procedure.
To achieve the above and other objects, the present invention includes a method for determining the prostatic boundaries, the rectum edge and the urethra on transrectal ultrasound images. An ultrasound image of a transverse cross-sectional outline of the prostate gland is acquired. For three-dimensional imaging, a series of images can be taken, one of each slice of the prostate. The initial ultrasound images are pre-processed to remove the noise and increase the contrast. The rectum edge is located from the bottom of the images. The key points on the prostate boundaries are located and connected under the iterative training of a knowledge-based model until the shape of the boundary reaches a stable state. The urethra is segmented near the center of the prostate.
In the present invention, the previously-used manual drawing to produce the prostate boundaries is eliminated, and the initial image produced by the ultrasound probe is automatically processed in a series of steps to create an accurate segmentation of the rectum edge, prostate boundary, and urethra edge. Then the algorithm will overlay the detected boundaries relative to the transverse cross-section of the prostate to provide important information for dosimetry planning.
The automated image segmentation of the present invention will reduce the median operative time for anatomy delineation for intraoperative planning, from 10 min. to 2 min. for complete outline of the prostate, urethra and rectum. The original image is provided by the ultrasound probe and a traditional stepper. The image is then processed to extract an outline of the prostate boundary.